Compressor blades for axial compressors are known extensively from the prior art. For example, EP 0 991 866 B1 discloses a compressor blade having a profile, the suction-side contour of which, at a suction-side intersection point with a reference line perpendicularly intersecting the profile chord at 5% of the length of said profile chord, has a curvature radius which is smaller than half the length of the profile chord. As a result, the effect to be achieved is that after a comparatively short stretch of circumflow around the blade airfoil on the suction side the velocity maximum is reached and the location of transfer of the flow from laminar to turbulent coincides with the location of the velocity maximum, as a result of which this profile has a particularly large operating range in which it efficiently compresses the gas flow.
Furthermore, it is known that so-called radial gas losses occur on the blade airfoil tips of compressor rotor blades. In this case, some of the pressure gain during operation of the axial compressor is lost as a result of a leakage flow being established across the blade airfoil tip from a pressure side of the blade airfoil to a suction side of the blade airfoil. In order to reduce this leakage flow, it is known that a radial gap, formed between the blade airfoil tips and an annular wall—lying opposite this—of the compressor duct, is always to be minimized as far as possible. Nevertheless, minimum values of gap dimensions must be maintained in this case in order to avoid rubbing of blade airfoil tips against the annular wall. This applies in this case particularly to transient operating states in which thermally induced expansions both of duct wall and of rotor blades are still not completed.
Moreover, it was frequently the case that the previous profiling of blade airfoil tips was adapted only to the specific inflow conditions in the region of the annular wall.
The actual profiling, however, was carried out without taking into consideration the actual three-dimensional flow effects on the blade airfoil tips. Conventionally designed blade airfoil profilings were therefore not optimally adapted to the complex flow conditions in the region of the blade airfoil tip. As a result, a notable improvement potential exists particularly in the case of compressor rotor blades with a small span and large relative gap depths (with regard to the span).
Since modern turbomachine blading arrangements, as known from EP 0 991 866 B1, have in the meantime achieved very high aerodynamic efficiency, there arises—with the tendency towards ever higher profile loads—an increasing proportion of the overall losses as a result these radial gap losses which occur in the outer region of the annulus close to the wall. A reduction of these appreciable losses therefore brings about a significant improvement of the efficiency of turbomachines and of axial compressors.
In order to reduce these radial gap losses, it is known from SU 1 751 430-A1, for example, to design the blade airfoil tip of rotor blades of an axial compressor according to the shape of an S. The camber line of the profile is formed by two opposed arcs which merge into each other at an inflection point. The inflection point is located in this case in the region of between 5% and 15% of the relative chord length. As a result of this, secondary flow losses and irregularities of the flow at the exit of subsonic compressor blades are reduced on account of the reduction of the pressure gradient. In particular, the pressure gradient in the front region and center region in the passages between the rotor blades is to be reduced in this case. According to SU 1 751 430-A1, the leading edge region is turned in the direction of the suction side of the blade airfoil, as a result of which the front, i.e. upstream, region of the profile has an inverse curvature in comparison to the rear, i.e. downstream, region of the blade profile.
In spite of the already existing solutions, there is still great interest in the reduction of radial gap losses of turbomachines in order to further increase the efficiency of these machines.